According to an article by W. H. Slabaugh, W. Dejager, S. E. Hoover, and L. L. Hutchinson in the Journal of Paint Technology, 44, 566, (1972), filiform corrosion occurs on aluminum when it is coated with an organic coating and the system is exposed to relatively high humidities. As the thickness of the coating increases, the rate of growth of the filiform corrosion decreases and the rate of transfer of water vapor from the atmosphere to the metal-coating interface decreases as the thickness of the coating increases. These two premises led to the experimental observation that moisture transfer across the organic film is necessary to support filiform corrosion. In addition to the already established use of anodizing the aluminum to increase the thickness of the oxide layer as a way to control filiform growth, the authors found that filiform corrosion is not likely to start if there are no discontinuities in the organic coating and the normal oxide layer is present. They also found that the presence of chloride ions or other such materials which react with the oxide layer is necessary for filiform corrosion to occur. Since the reduction of the relative humidity to values below 30 to 40% is not practical, the design of a less permeable coating was said to be a possible lead to less filiform corrosion.
The exposed portions of metal wheels on a speeding car are subjected to a fury of grit which abrades the protective coating. A hard and tough film is needed. The film should be non-porous initially and permanently and should retain superior adhesion to the metal to prevent the spread of corrosion at the interface of the film and metal. Visible pitting of the coating, reaching down to the metal surface, is not necessary for the growth of filiform corrosion, however.
Ultra-violet light absorbers and light stabilizers have long been used in coating formulations to retard light-induced yellowing, cracking, checking, and loss of gloss. In U.S. Pat. No. 4,226,763, Dexter et al. teaches some of the history of o-hydroxyphenyl-2H-benzotriazoles as UV absorbers and states that its 2-[2'-hydroxy-3', 5'-di-(.alpha., .alpha.-dimethylbenzyl) phenyl]-2H-benzotriazole exhibits outstanding efficacy in protecting organic coatings from degradation. In like manner, Rody et al. and Cortolano et al. teach about hindered amines and particularly polymeric polyalkylpiperidines as light stabilizers in U.S. Pat. Nos. 4,299,926 and 5,004,770, respectively. The use of a combination of such UV absorbers and light stabilizers in clear powder coating compositions is taught in U.S. Pat. No. 4,402,983. Said compositions are said to be useful as the clear coat over a color coat in an automobile finish. Thus, the use of UV absorbers and light stabilizers to prevent the cracking and checking of an organic coating is well known. As was said before, however, the appearance of filiform corrosion precedes any evidence of degradation of the organic coating.
The precise mechanism by which the present invention prevents filiform corrosion at the interface of an aluminum wheel surface and its protective coating is not fully understood. Protective clear coatings have been used in comparable film thicknesses on aluminum wheels before this invention. Such coatings have been observed to fail to prevent filiform corrosion over time even though the coating itself had not suffered visually unacceptable degradation.